(Per)fluoropolyether (PFPE) lubricants are well-known engineered lubricants combining high-temperature performance, nonflammability, thermal and oxidative stability, and chemical inertness under a wide range of conditions.
For applying PFPE lubricants on sliding/moving parts, said lubricants can be applied as neat compounds. Nevertheless, kinematic viscosity of these fluids is such that their application on surface of difficult accessibility can be improper and/or their metering might be uneasy to control, so that uneven or inappropriate distribution of the lubricant is obtained.
Thus, in certain cases, to solve above mentioned issues, it has been suggested to dilute/suspend the same in suitable fluids so as to obtain a PFPE lubricant-containing composition having a substantially modified kinematic viscosity, so that different techniques, including dipping, spin coating, spraycoating, and the like, becomes suitable methods for conveying said PFPE lubricant to the surface to be lubricated.
Thus, JP 59107428 (SUWA SEIKOSHA KK) of Jun. 21, 1984 discloses a method of forming a lubricating film on a surface, wherein a perfluoropolyether lubricant is dissolved in a halogenoalkane solvent such as perfluoroalkane or chlorofluoroalkane, so obtained solution is then applied to the target surface by means of dipping, spin coating, spray coating or the like, final drying yielding the lubricating film. Nevertheless, in this method, use is made of fluorinated solvents like CFC 113, whose use has raised serious environmental concerns.
Certain low boiling point halogenated solvents have also been proposed for conveying PFPE lubricants; nevertheless, an appropriate recovery of the fluid during drying/evaporation is generally required, due to above mentioned environmental concerns.
Alternative solutions avoiding the use of such low boiling point halocompounds have been suggested; in particular, U.S. Pat. No. 5,149,564 (HITACHI LTD) of Sep. 22, 1992 teaches the use of water-borne emulsion comprising the PFPE lubricant and a suitable surfactant, preferably a polyoxyethylene alkylether surfactant for forming lubricating film on surfaces. Nevertheless, only very diluted emulsions possess suitable stability against phase separation for being actually used at industrial level, so that extensive drying is required, with consequent limitation of real applicability of this solution.
Aqueous based PFPE microemulsions such as those taught in U.S. Pat. No. 4,990,283 (AUSIMONT SPA (IT)) of Feb. 5, 1991 or in U.S. Pat. No. 5,211,861 (AUSIMONT S.R.L.) of May 18, 1993 might also provide an effective way of conveying the exceptional properties of PFPE lubricants to surfaces; nevertheless, these technologies requires use of fluorinated surfactants which might raise environmental concerns.
Nevertheless, none of the above detailed solution offers a reliable method for forming adequate lubricating films, in particular on vertical surfaces.